Shatterpoint by Matthew Stover
Author Matthew Stover explores the psychology of Jedi Master Mace Windu as he returns to his home planet to find his missing former apprentice and face the darkness in his own soul. Inspired by the real-world film Apocolypse Now and the classic (non-Star Wars) novel Heart of Darkness.

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This is a prequel era book from the Legends timeline. It may be contradicted by newer books, movies, or TV shows (if so, just think of it as an alternate universe).